Holocene Climate Changes Recorded By Soil Sediments In Tianshui Basin

The Holocene was an important period for the development of human civilization.The study of climatic evolution during this period has became the focus of paleoclimate research.Tianshui was located at the intersection of the Tibet Plateau and the Loess Plateau,and at the apex of the "Monsoon Triangle".It was a typical area with unique geographical location in the study of paleoclimatic climate.The loess was deep and complete,and it was an ideal research material for recording the environmental changes of loess soil.The information on the large number of paleoclimatic environments contained in these loess-paleosol sequences remains to be further explored.Many problems were still unclear,such as the stratigraphic sequence and chronological sequence of these loess,physical and chemical properties and sedimentary characteristics and weathering characteristics.This paper not only shed light on weathering intensity,stratigraphy and chronology frame,also had the profound significance to the climate fluctuation under the background of monsoonal climatic change.Mainly obtained the following conclusions:(1)Established a sequence of loess layers.The stratigraphic sequence of the loess of the Shijiaya section was from top to bottom:topsoil(TS)?Holocene loess(L0)?upper part of paleosol(S01)?weak loess(Lx)?lower part of paleosol(S02)?Transitional Loess(Lt)? Manlan Loess(L1).(2)Sedimentological characteristics of loess:The magnetic susceptibility,particle size,loss on ignition,hygroscopic water and calcium carbonate index of each layer in the SJY section were significantly different.The low-frequency magnetic susceptibility of the SJY profile varied from 70.7×10-8 to 119.4×10-8m3/kg,with an average value of 94.39×10-8 m3/kg,and the highest value appeared on S01,which was 119.4×10-8m3/kg.The minimum value of Lx was 76.3×10-8 m3/kg.The particle size composition of SJY profile was mainly coarse silt.According to the particle size parameter,the sorting property was better,the kurtosis state was moderate,and the skewness was positively skewed.Clay,fine silt,clay/coarse ratio and magnetic susceptibility had high values in the paleosol layer;the trend of moisture absorption and loss on ignition in the profile was basically consistent with the trend of magnetic susceptibility,which was higher in the paleosol layer and lower in the loess layer.(3)Geochemical characteristics of loess:The variation trends of oxides SiO2,Fe2O3,K2O and Al2O3 in the profile were basically the same,both higher in S01 and S02,lower in loess layer.The weathering of the paleo-soil layer was strong,and the weathering of the loess layer was weak.Si,Fe,K and Al were relatively enriched,while Na and Ca occured to varying degrees.(4)Weathering characteristics of loess:The loess-paleosol sequence of the SJY section experienced a weak chemical weathering process as a whole.During the formation of paleosol,the weathering into soil is relatively strong,and the weathering of loess during the formation of loess is weak.(5)The climate was cool in early Holocene,generally humid and warm in the Middle-Holocene,and then became no stable with climatic change at 7000?6000a B.P..In the late Holocene,the climate entered a relatively cool and rainy period.(6)In the mid-Holocene,the magnetic susceptibility and the grain size curve showed the peak-to-valley interlacing,especially at 110?80 cm,the clay particle content curve fluctuated sharply during this period.The fluctuation of each element and element parameter fluctuateed sharply during this period.The stability elements such as Si and Fe were significantly reduced,and elements such as Ca and Na were enriched.It indicated that the summer monsoon was unstable and the intensity changed frequently.The climate was repeated,Climate fluctuations occurred at 7000?6000 a B.P.,reflecting the characteristics of the millennial climate shock".